Conventional wall box cover plates, referred to herein as face plates, are usually in the form of a flat plate having one or more openings providing access to a wiring device within a wall box. The purpose of the face plate is to provide a decorative cover for the wall box installation while preventing operator exposure to the interior of the wall box which contains electrical wiring. As used herein, "wiring device" refers generally to electrical power control/distribution devices, including but not limited to ON/OFF switches, outlets, dimmers, motor speed selector switches and the like, and to certain communications/data connectors such as telephone jacks, coaxial cable connectors, TV antenna connectors, and computer network cable connectors.
Wiring devices conforming to NEMA standards include a metal yoke or mounting strap with oversized mounting holes which permit horizontal position adjustment of the wiring device within a wall box. The yoke strap also includes one or more threaded mounting holes for attaching the face plate by screw fasteners onto the yoke strap, thereby completing the enclosure of the device and wiring terminations within the wall box.
The enclosed wiring device may have a switch actuator, control knob, power receptacle or the like which is supported on the mounting strap by an insulating escutcheon. The face plate access opening is closely conformed about the escutcheon to limit operator access to electrical terminals and wiring within the wall box.
Such wiring devices are conventionally mounted by screw fasteners onto wall boxes as described in NEMA Standards OS-1 and OS-2. Conventional face plates are also mounted by screw fasteners onto the wiring device. The standards for mounting the face plate are described in NEMA standards WD-1 and WD-6.